//可以指代任何类型
let a: any;
let b: unknown;

a = 1;
a = '';
a = [];
a = {};
a='hello';
console.log(a.length)

b = 1;
b = '';
b = {};
b = [];
b ='hello';
//类型“unknown”上不存在属性“length”
//在使用unknown类型之前需要明确告知类型 
console.log((b as string).length)